Fan apparatus

ABSTRACT

An improved axial-flow unshrouded type of fan suitable for atomizing streams of liquid impinging on the leading edges of the rotating blades near the hub includes a first set of many equally spaced blades and a second set of an equal number of shorter blades alternating with and approximately equally spaced between the blades of the first set. Each blade of the first set has a ratio of blade length to chord length (aspect ratio) such that a liquid stream impinging on the leading edge near the hub will spread out across the blade for substantially even distribution in atomized form from the entire length of the trailing edge. The two blade sets are arranged axially so that the swept path of the first set substantially overlaps the swept path of the second set, but at least a portion of each blade of the second set extends upstream of a corresponding portion of each blade of the first set. The blade length of each blade of the second set is sufficiently shorter than the balde lengths of the first set, and the circumferential spacing between adjacent blades of the first and second sets is sufficiently close so that the flow of air from the second set of blades approximately cancels the back flow of air from the first set of blades.

United States Patent 1 1 Ericson et al.

[ FAN APPARATUS [76] Inventors: David B. Ericson, 625 S. Broadway,

Nyack; Goesta Wollin, Snedens Landing, Palisades; Roger L. Zaunere, 62 N. Greenbush Rd., W.

Nyack, all of NY.

[22] Filed: Sept. 2, 1971 [2 1] Appl. No.: 177,271

Related U.S. Application Data [60] Division of Ser. No. 834,225, June 11, 1969, Pat. No. 3,610,527, which is a continuation of Ser. No. 741,512, July 1, 1968, abandoned,

[52] U.S. Cl. 416/201 [51] Int. Cl. F04d 13/12 [58] Field of Search 416/200, 201, 198

[56] References Cited UNITED STATES PATENTS 1,021,822 4/1912 Broussouse 416/201 X 1,607,116 11/1926 Checkley 416/201 1,889,717 11/1932 Warfel 416/200 1,946,571 2/1934 Briner 416/201 X 2,938,662 5/1960 Eckert et al. 416/201 X 3,193,185 7/1965 Erwin et al. 4l5/DIG. 1 3,606,579 9/1971 Mehus 416/201 X 2,339,602 l/l944 Hagen 416/201 FOREIGN PATENTS OR APPLICATIONS 1,012,041 4/1952 France 416/198 1,919,086 12/1970 Gennany 416/201 Primary Examiner-Everette A. Powell, Jr. Attorney-Kenyon et a1.

[57] ABSTRACT An improved axial-flow unshrouded type of fan suitable for atomizing streams of liquid impinging on the leading edges of the rotating blades near the hub includes a first set of many equally spaced blades and a second set of an equal number of shorter blades alternating with and approximately equally spaced between the blades of the first set. Each blade of the first set has a ratio of blade length to chord length (aspect ratio) such that a liquid stream impinging on the leading edge near the hub will spread out across the blade for substantially even distribution in atomized form from the entire length of the trailing edge. The two blade sets are arranged axially so that the swept path of the first set substantially overlaps the swept path of the second set, but at least a portion of each blade of the second set extends upstream of a corresponding portion of each blade of the first set. The blade length of each blade of the second set is sufficiently shorter than the balde lengths of the first set, and the circumferential spacing between adjacent blades of the first and second sets is sufficiently close so that the flow of air from the second set of blades approximately cancels the back flow of air from the first set of blades.

6 Claims, 8 Drawing Figures FAN APPARATUS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a division of application Ser. No. 834,225 filed June 11, 1969 and issued as U.S. Pat. No. 3,610,527 on Oct. 5, 1971 which, in turn, was a continuation-in-part of application Ser. No. 741,512, filed on July 1, 1968, now abandoned, by the same inventors and entitled Atomization Apparatus And Method.

This invention relates in general to atomization and more particularly to a method and apparatus for efficiently making snow under a variety of ambient temperature conditions.

BACKGROUND OF THE INVENTION erable attention because of the increased interest in skiing and the requirement that a profitable commercial establishment not be dependent on the happenstance of snow falling from the skies. The atomization technique which has become most commonly used in snow making involves the atomizing of water forced through a nozzle by the use of compressed air. This fairly widely used technique has a number of disadvantages. The small openings of the nozzle tend to freeze up. The use of compressed air requires considerable power which must be made available at the site where the now is being laid. The wide variation in the drop size emitted by a nozzle results in less than the complete conversion of the water to snow thereby resulting in the problem of creating considerable undesirable ice; I

In general, known atomization techniques require considerable use of energy to achieve the desired atomization. For example, the use of compressed air to force the liquid through a nozzle results in a great deal of wasted energy. Some atomization techniques also require complex and expensive equipment as, for example, where electrostatic atomization techniques are employed. Even rotary disc atomization requires relatively large amounts of energy for the amount of atomization obtainable. Because of the energy cost, equipment cost and equipment complexity considerations, known atomization techniques have had limited uses. In general, they cannot be used for large scale atomization of a fluid such as would be required in the process of separating, for example, salt from sea water.

Accordingly, it is a major purpose of this invention to provide a more efficient technique for the atomization ther on the equipment or on the ground.

It is another major purpose of this invention to provide an atomization and snow making technique which will be substantially more efficient in its use of power than presently known techniques.

It is a further specific purpose of this invention to provide a snow making technique which can be employed over a wide range of ambient temperatures and which will use water having a wide range of water temperatures including water temperatures well above freezing.

It is a further specific objective of this invention to provide a relatively uniform drop size of the atomized particle.

BRIEF DESCRIPTION OF THE INVENTION In brief, this invention broadly involves the flow of liquid over'the surfaces of rotating fan blades or the like in order to provide an atomized spray off the trailing edge of each fan blade. The spread out of liquid over the surface of the fan blades and the evaporation of the liquid, due to the air flow over the liquid, as it spreads out over the fan blades, results in a sufficiently thin moving film of liquid being fed across the sur-face of the fan blades to the trailing edge. Thus when this film of liquid leaves the trailing edge of the fan blade, the result is a finely atomized sprayof droplets.

A fan having a hub and a relatively large number of fan blades (for example, 16 through 48 fan blades have been found useful) is mounted for rotation, usually in a horizontal plane about a vertical axis through the center of the hub. A stream of liquid, such as water, in the form of a series of jets arranged in an annular ring around the hub of the fan, is thrown up at the rotating fan blades near the inboard section of the fan blades. The leading edges of the fan blades cut into the jets of water causing the water to spread out across the fan blades, preferably along both the top and bottom surfaces of each fan blade. Centrifugal forces cause the primary path of waterv flow to be radially outward. However, the flow of air over the surface of the fan blades gives the flowing film of water a circumferential component. The relationship between the length and width of the fan blades is preferably selected to be such that the bulk of the liquid involved comes off the fan blade at the trailing edge thereof rather than off the outboard tip.

Where an upward draft of air is desiredin order to cause the atomized liquid to move up and away from the fan blades, the fan blades are given an appropriate pitch to cause an updraft of air. Under such circumstances, which is particularly important when making snow, half of the blades may be displaced downward somewhat from the other half of the blades so as to form two sets of blades rotating in two parallel planes. Those blades in the lower plane of rotation are preferably made shorter than the blades in the upper horizontal plane. As a result most of'the undesirable turbulance is avoided and an updraft of air is created that can carry all of the atomized liquid away from the fan blades and associated equipment.

For snowmaking, in particular, it is important that the water be cooled by evaporation to a temperature well below freezing so that the particles freeze before they hit the ground. It is believed that this is a main reason why the surface area of the fan blades used for snowmaking ahould be as large as possible.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and purposes of this invention will be come apparent from the following detailed description and drawings, in which:

FIG. 1 is a perspective view of a first embodiment of this invention adapted for snow making;

FIG. 2 is a plan view of the FIG. 1 device;

FIG. 3 is a cross sectional view along the plane 33 of FIG. 2;

FIG. 4 is a mechanical schematic illustrating the relationship between the jets of fluid and the blades in motion;

FIG. 5 is a plan view of a second embodiment of this invention, which embodiment is preferred, over the FIG. 1 embodiment, for snow making;

FIG. 6 is a plan view of the template (flat, two dimensional model) from which the smaller blades in the FIG. 5 embodiment are made;

FIG. 7 is a plan view of the template from which the larger blades in the FIG. 5 embodiment are made; and

FIG. 8 is an elevation view in partial cross-section of the FIG. 5 embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In General The figures illustrate preferred embodiments which have been developed for the production of snow.

As may be seen in FIG. 1, the atomizing device 10 is shown mounted on a stand 11. The device 10 includes a hub 12 to which are attached a relatively large number of blades 14. A shaft 16 is coupled to an electric motor 18, the motor being inside a casing 20, so that the hub 12 and blades 14 may be rotated. An electric power line 22 is shown for providing power to the motor 18. The casing is mounted on the stand 24 so that the snow that is made is raised somewhat from the ground and thereby can be better distributed over the surrounding area. In snow making, apparatus 10 should be pivotally mounted on the stand 11 so that the apparatus 10 can pivot in a vertical plane. This will provide directional distribution of the snow.

Water is supplied through a hose 26 into a manifold 27 within the casing 20 (see FIG. 3). The water is supplied under sufficient pressure so that is is emitted from the openings 28 in jets which are directed to the inboard end of the blades 14. As shown in FIG. 1, the openings 28 are arranged in an annular fashion in the casing 20. These openings 28 are disposed at a radius such that the jets of water emitted from the openings 28 will impinge on the blades 14 at or near the inboard section of the blades 14. This relationship between the openings 28 and the blades 14 may best be seen in FIGS. 3 and 4.

The Blades As may best be seen in FIGS. 2 and 3, the blades 14 are, in the preferred snow making embodiment, composed of two sets of blades. One set of blades are the longer blades 14a and the other set of blades are the shorter blades 14b. The shorter blades 14b are below the longer blades 14a. In a presently preferred embodiment, l6 blades 14 are employed, eight of which are the longer blades 14a and eight of which are the shorter blades 14b. A 5 inch blade length for the longer blades 14a and a 3 74 inch blade length for the lower blades 14b has been found useful. In this preferred embodiment, there is a 1 inch axial displacement between the set of longer blades 14a and the set of shorter blades 14b. In this particular embodiment, the blade width is approximately l 5 1 inches.

In the embodiment shown, and with reference to FIG. 1, the blades 14 are rotated in a counter clockwise direction looking down at the blades. In order to cause the atomized particles leaving the trailing edge of the blades to be thrown away from the device 10 (so that the snow will be distributed on the desired ground area), the blades 14, as may best be seen in FIG. 4, are made with a moderately steep pitch of approximately 35 from a horizontal plane. Specifically, the chord connecting the leading and trailing edges of the blades 14 is at an angle of approximately 35 with a plane perpendicular to the axis of rotation of the device 10. The blades 14 have a slight concave upward upper face to facilitate the throwing off of the atomized particles from the trailing edge of the blade 14.

As shown in FIG. 4, the hub 12 is rotating such that the blades 14 are moving to the right. Water (shown by arrows) is thrown up from the openings 28 and impinges upon the blades 14. Because the leading edges of the blades cut into the stream of water, water tends to flow over both the upper and lower surfaces of the blades 14. The water is then thrown off as atomized particles from the trailing edges of the blades 14 in the direction indicated by the arrows at the trailing edges of the blades 14.

Where it is not necessary to create as great an air updraft to carry off the atomized particles, lower pitched blades would be preferable in order to reduce the power requirements.

When employed to atomize water, the atomization device 10 atomized 15 gallons per minute of water by use of a 5 horsepower motor 18 to rotate the blades 14 at 3,500 revolutions per minute (rpm). The resultant water spray provided atomized droplets of water of approximately 500 microns in diameter. By observation, the distribution of droplet sizes around the 500 microns was relatively uniform compared with the distribution of droplet sizes achieved when compressed air is used to force water through a nozzle.

Although the dual level of blade arrangement is not essential to achieve atomization, it does achieve a very important result when the device of this invention is used for making snow. The result that this dual level of 4 blades provides is that there is an increase in the assurance that percent snow will be developed and that all the snow is thrown away from the snow making device 10.

It is believed that the reason why the dual level of blades operates more effectively and efficiently than does a single row of blades relates to the effect of the blade arrangement shown on the turbulance that is normally generated off the trailing edge and the tip of the blades. The shorter set of blades 14b generates an updraft which appears to cancel out the turbulance and downdraft (or back flow) characteristics which would be observed if only one set of blades in one rotational plane were employed. The lower and shorter set of blades 14b does not generate a significent back flow or downdraft turbulance of its own presumably because the upper set of blades 14a produces a sufficiently strong updraft of air so that the air flowing from underneath all of the blades 14a and 14b cancels out the turbulance or downdraft that might occur if the shorter set of blades 14b were used alone. However, to achieve this result, it has been found necessary to make the upstream set of blades 14b shorter than the downstream I of blades 14a. If the lower set of blades 14b were extended to have a length equal to the upper set of blades 14a, then the lower set of blades 14b would exhibit at their outborad end some of the back flow or downdraft which would result in throwing or dropping atomized particles down toward the apparatus 10.

Operation of the Invention In developing the FIG. 1 device, various equivalent devices were tested. Two of these tests indicate the efficiency and effectiveness of the device of this invention as broadly conceived.

One such test involved a fan having 48 blades, 24 of the blades being in an upper rotational plane and 24 of the blades being in a lower rotational plane. This fan was rotated at 2800 rpm and was able to atomize water fed to the fan blades at a rate of 6 gallons per minute. This test was run for a 2 hour period. The output was 100 percent snow in this test where the water temperature was 43F., the air temperature ranged from 28 and 31F, and the ambient humidity was 42 percent. Under these conditions, a 7 horsepower input was adequate to handle an intake of 6 gallons of water per minute and provide an output that contained no ice.

A second test employed a fan having 24 blades, all in one rotational plane, operating at 2800 rpm produced 100 percent snow from 12 gallons of'water per minute. However, in the second test the water temperature was at 35F, the air temperature was at 24F., and the relative humidity of the air was 59 percent. To achieve this result a 5 horsepower input was required. Because a single level of blades was employed, the distribution of the snow was less than ideal.

In both of the above tests, the water was fed to the blades by a technique other than that shown in the drawings. The water feed was up through a hollow shaft into the base of a cup-like hub, then along the surface of the cup, up the inside surface of the cup, over the edge of the cup onto blades attached to the edge of the cup. The water feed illustrated in the figures is preferred, since it requires less power and increases the efficiency of the device.

Preliminary tests indicate that the device illustrated can produce 100 percent snow, at an ambient temperature as high as 27F., with a water input of gallons per minute while requiring only 5 horsepower. The major reason for this increased efficiency arises from the manner in which the water is fed to the blades 14. The annular feed, in lieu of shaft feed, avoids the need to have extra bearings, gearing and/or pulleys. This annular feed anables use of the pressure of the water over a disc or cup develops.

From'the above tests it may be seen that the device of this invention has a wide range of operating capabilities. When used to make snow, snow may be made from water having temperatures substantially above freezing and may be made in an atmosphere having a temperature substantially above that necessary for the creation of'snow. This is because the operation of this invention is such that where the cooling effect is optimized, the water temperature can be brought to within the 10F. (I2C.) to 15F. (lOC.) range where atomized particles will be converted to snow. In order to make snow by'a rapid process, the temperature of the atomzied droplets must be brought down to a maximum temperature somewhere between 10F. and 15F. Such a temperature is required to bring about the rapid conversion of a liquid droplet to snow.

Part of the reason for the efficiency of this rapid conversion to snow of the water applied to the fan blades is because of the extensive amount of evaporation that occurs as air flows over the film of water on the fan blades and, it is believed, because of further evaporation from the atomized particles of water thrown off the trailing edge of the fan blades. This further atomization occurs in the low pressure area that is formed around the trailing edge of the fan blades. When it is realized that the heat of vaporization of one gram of water is sufficient to cool 544 grams of water by 1C., it can'be appreciated that the extensive vaporization afforded by means of this invention will provide the results described.

The Figure 5 Embodiment After development of the FIG. 1 embodiment, further experimentation developed an improved device for snowmaking. This improved device, shown in FIGS. 5-8, provides greater assurance of producing effectively percent snow.

The two significant distinctions between the FIG. 5 embodiment and the FIG. 1 embodiment that provide the increased assurance of 100 percent snoware: (a) the larger fan blade area, through use of wider fan blades, in the FIG. 5 embodiment, and (b) the inboard portion of the leading edges of the longer blades is brought upstream to the same vertical level as are the leading edges of the shorter blades. Otherwise the two embodiments shown are very similar and much of the above description applies to the FIG. 5 embodiment.

As shown in FIGS; 5-8, the hub 112 and I6 blades 114 are mounted for rotation by a power driven shaft 116. Water supplied through a pipe 126 circulates through an annular manifold 127 and is emitted through a dual set of circular openings 128a, 1281;. The inner set of openings 128a are annularly disposed and concentric within the outer annularly disposed set of openings 12812.

The openings 128 are positioned close (about 1 inch in the embodiment tested) to the leading edges of all 16 blades 114. This arrangement assures that all the water is picked up by the blades and avoids a problem of a small percentage of water in large droplets falling near the machine as water and freezing on the ground into a sheet of ice. To make sure that allthe water is picked up on the fan blades the inboard portions of the leading edges of the upper (and longer) blades 114a are brought down to the level of the lower (and shorter) blades 1l4b. As may be seen from the FIG 6 and 7 templates, the inboard width of the template 114a for the longer blade is greater than the outboard width of the long blade template 114a and also greater than the inboard width of the short blade template ll4b'. However, the longer blades 114a are otherwise axially displaced upward from the shorter blades in the fashion described in connection with the FIG. 1 embodiment.

A second important feature of the FIG. 5 blades is their width. Instead of a blade width of 1 of an inch (as in the FIG. 1 embodiment), these blades 114 have a width between 2 A and 3 inches. The FIG. 5 blade 114 lengths are the same as the FIG. 1 blade 14 lengths. As a result, total blade surface area in the FIG. 5 embodiment is about 280 square inches. This larger surface area increases the evaporative cooling effect with the result that 100 percent snow is assured at higher air temperatures.

Blade 114 length and pitch are as described in connection with the FIG. 1 blades 14. FIGS. 6 and 7 illustrate the actual length and width dimensions in a tested model of this FIG. embodiment. The plates actually used were cut from the flat sheet of metal employing a template having the dimensions indicated in FIGS. 5 and 6. The flat sheet plate forms were then formed into plates having the pitch and curvatures discussed in connection with the plates employed in the FIG. 1 embodiment.

Five separate eight hour tests were made with the FIG. 5 embodiments. These tests show that this invention, employing only a 5 horsepower electric motor drive can produce essentially 100 percent snow from 8 to 9 gallons of water per minute using water at 35F. (2C.) at an air temperature of about 27F. (3C.), humidity below 70 percent and with no wind. As air temperature decreases, the amount of water that can be employed to provide 100 percent snow increases up to about 17 gallons of water per minute at 0F. (l8C.).

It might be noted here that in describing tests and the invention in general, it has been stated that lOO percent snow is created and that all of the water applied to the fan blades is converted to snow. Of course this is not literally so since some of the water is evaporated and converted to vapor. What is intended to be meant by such language, which language is in conformity with the usage in the art, is that no liquid water is formed. Thus when 100 percent snow is formed, no ice or layer of ice is formed because no liquid droplets of water strike the ground.

However, from a practical point of view, 100 percent of snow need not be formed in order to provide a satisfactory terrain for skiing. Thus, if desired, the rate at which water is fed to the fan blades may be increased above that which will provide 100 percent snow so that the snow created is only 90 percent or 80 percent of the atomized droplets that fall. Such a practice is obviously within the scope of this invention and might be desirable in order to assure that the maximum snow output is obtained where the sacrifice of creating a small amount of ice is tolerable.

This atomization device has been described in detail in connection with the preferred embodiment for making snow. Obviously, not only may various changes be made in connection with the embodiment for making snow without departing from the scope of this invention but various changes in both apparatus design and application may be made where the invention is applied to other than snow making without departing from the scope of this invention.

For example, the feeding of a solution to the device described will result in an efficient generation of atomized particles of the solution. If this is done at a sufficiently high ambient temperature, the result will be that the particles will be vaporized in the atmosphere and the dissolved material will drop out and can be collected. Under such conditions, the pitch of the blades may well be much less than that shown since it is not desired that the air flow be as great.

in general, there is a trade off between such parameters as rotational speed, ambient temperature, ambient humidity, size of atomized particles and quantity of fluid atomized. The optimized relationship between these parameters will depend upon the application involved and the given conditions within which the application must operate. It should be recognized that to achieve the desired amount of atomization, the layer of fluid flowing over the fan blades must be kept thin. lf larger droplets are usable then the layer of water can be thicker and the amount of fluid atomized can be increased. Alternatively, a faster rotational speed of the fan blades may result in a faster movement of fluid over the surface of the fan blades so that a greater quantity of fluid can be atomized without increasing the thickness of the sheet of fluid flowing over the fan blades.

The most efficient use of the invention for separation may call for a greater number of blades than illustrated to provide a greater total atomizing edge. With greater rotational speed, the larger number of blades may then enable the device to handle a greater flow rate of fluid.

The invention has been described in connection with atomizing a liquid either with or without a significant amount of dissolved material therein. However, the invention can be used with a slurry or suspension to achieve atomization and separation of the liquid and solid phases. Accordingly, the term liquid or fluid as used in the claims herein shall be understood to include a slurry or a suspension.

Although the method of feed shown is preferred, especially for snow-making, the feed can be widely varied. For some purposes, a gravity feed down onto the blades might be usable.

The dual level of fan blades 14a and 14b has been described in connection with the snow making embodiment as a preferred way to avoid drop off of snow on and near the equipment. However, the basic reason for this result is that the dual level blade design reduces edge turbulance and back flow. The resultant more efficient air flow may have applications in devices other than for snow making or even than for atomizing. The fan design described is in itselfa new and more efficient device for moving air.

The blades 14 have been discussed above as fan blades because, in the preferred embodiments, they are used to move the air in a given axial direction. This function is particularly important in snow-making and is also useful in other possible applications. However, broadly speaking, the blades 14 need not be designed to have the curvatures that are typical of fan blades. Thus is should be understood in the claims that the reference to blades includes blades having flat surfaces, as well as those blades whose curvature is designed to optimize the axial flow of air.

Accordingly, it should be understood that the following claims are intended to cover the various inventions of this design in all the embodiments and variations that would be obvious to those skilled in these arts.

What is claimed is:

1. In an unshrouded axial-flow fan having a rotatable hub, a plurality of blades extending outward from the hub, and means for rotating the fan within a predetermined design speed range, the improvement comprising:

a first set of many blades equally spaced about the hub, each blade of the first set having a ratio of blade length to chord length such that the bulk of a liquid film formed on each blade rotating within the design speed range of the fan by a stream impinging on the leading edge near the hub will flow across the blade and be distributed in atomized form from substantially the entire length of the trailing edge, and a second set of an equal number of blades alternating with and approximately equally spaced between the blades of the first set, the swept path of the first set substantially overlapping the path of the second set but at least a portion of the leading edge of each blade of the second set extending upstream of a corresponding portion of each blade of the first set, the blade length of each blade of the second set being substantially shorter than the blade lengths of the first set, yet the blade lengths of the second set being sufficiently long and the circumferential spacing between adjacent blades of the first and second sets being sufficiently close so that the flow of air from the second set of blades approximately cancels the back flow of air from the tips of the first set of blades. 2. The fan of claim 1 wherein the leading edges of said longer subset of fan blades curve upstream to an inboard section having essentially the same axial location as the inboard section of the leading edges of said shorter subset of fan blades.

3. The fan of claim 1 wherein the inboard width of the blades in said longer subset of fan blades is greater than the outboard width of the blades in said longer subset of fan blades.

4. The fan of claim 1 wherein the width of the blades in said longer subset of fan blades is greater than the width of the blades in said shorter subset of fan blades.

5. The fan of claim 1 wherein chords connecting the leading and trailing edges of said fan blades make an angle of approximately degrees with a plane perpendicular to the axis of rotation of said blades.

6. The fan of claim 1 wherein the downstream faces of the blades are slightly concave, whereby the leading edges of said blades have a smaller pitch angle than the trailing edges of said blades. 

1. In an unshrouded axial-flow fan having a rotatable hub, a plurality of blades extending outward from the hub, and means for rotating the fan within a predetermined design speed range, the improvement comprising: a first set of many blades equally spaced about the hub, each blade of the first set having a ratio of blade length to chord length such that the bulk of a liquid film formed on each blade rotating within the design speed range of the fan by a stream impinging on the leading edge near the hub will flow across the blade and be distributed in atomized form from substantially the entire length of the trailing edge, and a second set of an equal number of blades alternating with and approximately equally spaced between the blades of the first set, the swept path of the first set substantially overlapping the path of the second set but at least a portion of the leading edge of each blade of the second set extending upstream of a corresponding portion of each blade of the first set, the blade length of each blade of the second set being substantially shorter than the blade lengths of the first set, yet the blade lengths of the second set being sufficiently long and the circumferential spacing between adjacent blades of the first and second sets being sufficiently close so that the flow of air from the second set of blades approximately cancels the back flow of air from the tips of the first set of blades.
 2. The fan of claim 1 wherein the leading edges of said longer subset of fan blades curve upstream to an inboard section having essentially the same axial location as the inboard section of the leading edges of said shorter subset of fan blades.
 3. The fan of claim 1 wherein the inboard width of the blades in said longer subset of fan blades is greater than the outboard width of the blades in said longer subset of fan blades.
 4. The fan of claim 1 wherein the width of the blades in said longer subset of fan blades is greater than the width of the blades in said shorter subset of fan blades.
 5. The fan of claim 1 wherein chords connecting the leading and trailing edges of said fan blades make an angle of approximately 35 degrees with a plane perpendicular to the axis of rotation of said blades.
 6. The fan of claim 1 wherein the downstream faces of the blades are slightly concave, whereby the leading edges of said blades have a smaller pitch angle than the trailing edges of said blades. 